High-frequency millimeter wave absorption of indium-substituted ε-Fe2O3 spherical nanoparticles (invited)

In this work, we prepared indium-substituted ε-iron oxide (ε-InxFe2−xO3) spherical nanoparticles by a combination method of reverse-micelle and sol-gel techniques. The powder X-ray diffraction pattern with Rietveld analysis shows that ε-InxFe2−xO3 has an orthorhombic crystal structure (space group: Pna21), and the In3+ ions mainly replace the Fe3+ ions at B site among the four nonequivalent Fe3+ sites (A–D sites). The magnetic measurements show that the coercive field (Hc) at 300 K decreases with increasing x, i.e., Hc = 21.9 kOe (x = 0), 12.2 kOe (x = 0.04), 11.6 kOe (x = 0.09), 7.8 kOe (x = 0.13), and 5.9 kOe (x = 0.18). Millimeter wave absorption was measured by terahertz time-domain spectroscopy, and the decrease of resonance frequency (fr) is observed, i.e., fr = 182 GHz (x = 0), 160 GHz (x = 0.04), 143 GHz (x = 0.09), 123 GHz (x = 0.13), and 110 GHz (x = 0.18). This decrease in the fr value is understood by the decrease of magnetic anisotropy, which is caused by the replacement of Fe3+ (S = 5/2) with nonmagnetic In3+ (S = 0) at B site contributing to the magnetic anisotropy.